Compositing Layers Have Halos
You have that problem because the alphas of each plate are correlated, as the foreground doesn't overlap the background, it masks it out.

Solving it is not super straight-forward but it's quite easy:


Add add the alpha channels of each layer together. Use a mix node with the "add" mode.
Add RGB plates together, same as above: mix node, "Add".
Use the "set alpha" node to assign the result of the added alphas to the result of the added RGB plates.
Profit :-)


The result will be properly combined with no halos or holes.